A power supply controller is conventionally provided, in which a high-power semiconductor switching element such as a power MOSFET is disposed on a current supply line connected between a power source and a load, and which is configured to control the power supply to the load by switching the semiconductor switching element between ON and OFF. In such a power supply controller, it is known that a self-protective function is provided for protecting its own semiconductor switching element. The self-protective function turns off the semiconductor switching element by controlling the potential of the control terminal (e.g., the gate in the case of a MOSFET) of the semiconductor switching element, when an overcurrent (i.e., an abnormal current) has occurred on the current supply line due to short-circuiting in the load, for example. Specifically, as shown in JP-A-2001-217696, a current detecting resistor (shunt resistor) is serially connected to the load terminal (e.g., the source or drain in the case of a MOSFET) of the semiconductor switching element, and a load current passing through the semiconductor switching element is detected based on the interterminal voltage of the resistor. If the load current value is larger than a predetermined threshold, an occurrence of an overcurrent anomaly is determined, so as to turn off the semiconductor switching element resulting in a shutoff state.
In regard to this configuration breaking of wire could occur, for example, on a current supply line connected between the load terminal of the semiconductor switching element and the load. The wire-breaking anomaly should be detected in some cases. For example, in the case that the above power supply controller is used for controlling power supply for a vehicle lamp in order to provide Daytime lighting (or Daytime running light), it is difficult for a vehicle driver to find in the daytime that the vehicle lamp fails, due to breaking of wire, to normally turn on. In this case, the wire-breaking anomaly is particularly required to be detected. For example, when breaking of wire has occurred on the whole or a part of the above current supply line, the load resistance will increase. Therefore, if the load resistance exceeds a predetermined value, a wire-breaking anomaly can be determined based thereon. A construction for this determination can be, for example, a construction in which a wire-breaking anomaly is determined if a load current falls below a fixed threshold. However, according to this construction, a load resistance value, based on which a wire-breaking anomaly is determined, varies with variation of the power supply voltage. This presents a problem that detection of a wire-breaking anomaly may fail to provide an accurate detection result. In view of this, the power supply voltage and the load current are conventionally measured, and a wire-breaking anomaly is determined by processing the measurements using a microcomputer.
The present invention was made in view of the foregoing circumstances, and an object thereof is to provide a power supply controller capable of detecting a wire-breaking anomaly without using a microcomputer.